[ossiblue]

The condition of the nipple is likely indicative of most of the remaining iron plumbing in the house. Due to its location, at the bottom of a vertical drop and connected to a hot water source that will accelerate electrolysis, the nipple is nearly rusted through. You can bet the rest of the plumbing, being 70 years old, has areas of sever rust occlusion as well, mostly in the hot water pipes. That being said, if you still want to only fix the immediate problem, you will need to be very careful not to destroy any weakened pipes further along the system.

One plus is that you are dealing with a 90* elbow which is very hardy and thick-walled. I would suggest you attempt to cut out the old nipple threads from the female threads of the elbow. This will be tedious work, but you might be able to do it. You can use a die to chase the threads in the elbow once most of the rusted nipple is cleaned away. Still, you may find the elbow cannot be salvaged.

From your post, it sounds like the 90 is protruding from the wall and the pipe to the attic, to which it is attached, is behind the wall. If that's correct, I don't see how you can remove the elbow without going into the wall to allow the 90 to rotate and to grip the pipe with a wrench to prevent it from turning as you unscrew the elbow. If you can do that, then that would be the approach, keeping in mind the pipe to the attic may be weak and you could wind up with even more replacement problems. Working on old plumbing is opening the lid to Pandora's Box and working on old galvanized plumbing is almost guaranteed to lead to unexpected difficulties.

Best of luck.